1. Field of the Invention
This invention relates generally to the field of thermal barrier coatings, and more particularly to a thermal barrier coating for a very high temperature application such as a combustion turbine engine. In particular, this invention relates to the field of multiphase ceramic thermal barrier coatings for high temperature application for coating superalloy components of a combustion turbine.
2. Background Information
The demand for continued improvement in the efficiency of combustion turbine and combined cycle power plants has driven the designers of these systems to specify increasingly higher firing temperatures in the combustion portions of these systems. Although nickel and cobalt based "superalloy" materials are now used for components in the hot gas flow path, such as combustor transition pieces and turbine rotating and stationary blades, even these superalloy materials are not capable of surviving long term operation at temperatures sometimes exceeding 1,200.degree. C.
Examples of cobalt or nickel based superalloys are, for example, Cr.Al.Co.Ta.Mo.W, which has been used for making SC turbine blades and vanes for gas turbines, as taught, for example, in U.S. Pat. No. 5,716,720 (Murphy). These turbine components are generally protected by a basecoat of MCrAlY, where M is selected from the group of Fe, Co, Ni, and their mixtures, as taught for example, by U.S. Pat. Nos. 5,763,107 and 5,846,605 (both Rickerby et al.) and by U.S. Pat. Nos. 4,916,022; 5,238,752; 5,562,998; and 5,683,825 (Solfest et al.; Duderstadt et al.; Strangman; and Bruce et al., respectively). These basecoats are usually covered by an aluminum oxide layer and a final thermal barrier coating ("TBC"). The standard thermal barrier coating, however, is made from yttria-stabilized zirconia, ceria-stabilized zirconia, scandia-stabilized zirconia or non-stabilized zirconia, as taught, for example, by U.S. Pat. No. 5,683,825 Bruce et al. patent. A particularly useful state of the art TBC is 8 wt. % yttria stabilized zirconia ("8YSZ").
Many of the ceramic thermal barrier layers are deposited as a columnar structure in the direction of the coating layer thickness, as taught in U.S. Pat. Nos. 4,321,311 and 5,830,586 (Strangman and Gray et al., respectively). This structure can be formed by electron beam physical vapor deposition ("EBPVD") as in Bruce et al. U.S. Pat. No. 5,683,825, or a combination of electron beam deposition and ion beam irradiation, or the like, such as the ZrO.sub.2 thermal barrier layer taught in U.S. Pat. No. 5,630,314 (Kojima et al.). Strangman U.S. Pat. No. 5,562,998, additionally vapor infiltration or sol-gel coats the columnar grains with a submicron thick layer of unstabilized zirconia or unstabilized hafnia, functioning as a bond inhibitor between the discrete columns.
Modern gas turbine engines can achieve higher efficiencies by increasing the turbine inlet temperatures. This subjects the TBCs to high temperatures. TBC materials that are phase stable at high temperatures upon long term exposure will be required. The current state-of-the-art electron beam physical vapor deposited ("EBPVD") 8 YSZ coatings destabilize above approximately 1200.degree. C. In addition, the long term high temperature exposure leads to potential sintering and loss of strain compliance, and possible premature TBC failure. 8YSZ coatings are also susceptible to corrosion upon exposure to contaminants in the fuel and erosion due to foreign object damage. Therefore, some of the key requirements for new TBC candidates for high temperature applications are high temperature phase stability, a reduced tendency to sinter, good corrosion and erosion resistance, all of them to be maintained upon long term exposure. These requirements are in addition to the primary needs of a TBC, such as, a low thermal conductivity with minimal coefficient of thermal expansion mismatch with the superalloy substrate.